Brake actuators for vehicular air brakes have a power chamber into which air is supplied to apply a pressure against a diaphragm. The diaphragm in turn forces a brake rod to apply the brakes. In a correctly adjusted brake system, the rod should not travel more than about two inches to apply the brakes. For example, one inch of rod travel translates into 0.020 inches of brake lining wear. Excessive rod travel reflects worn linings, or damaged drums. In either event it is an unsafe condition, and when found by law enforcement agencies, results in citations, and even holding the vehicle until after repairs are made.
It is an object of this invention to provide a sensor that can be attached to the brake actuator and to the rod which can sense an give warning when the rod extension exceeds a maximum safe limit.